1. Field of the Invention
The present invention relates generally to optical modulation in wavelength division multiplexing, and more particularly to a method, device, and system for optical modulation applied to wavelength division multiplexed light including a plurality of optical carriers having different wavelengths.
2. Description of the Related Art
In recent years, processing of massive amounts of information has been needed with development of an advanced information society, and optical fiber communications fit for a large capacity have been applied to a transmission network for transmitting information. While a transmission rate of information in optical fiber communications has already reached 2.4 Gb/s or 10 Gb/s, a further increase in transmission capacity will be needed in a motion picture captured communications system that is expected to be put to practical use in the future.
Wavelength division multiplexing (WDM) is known as one of the techniques for increasing a transmission capacity in optical fiber communications. In WDM, wavelength division multiplexed light including a plurality of optical carriers having different wavelengths are used. By individually modulating the optical carriers, a transmission capacity per channel of an optical fiber transmission line is increased according to the number of WDM channels.
In the case of carrying out WDM, it is sometimes required to perform optical modulation of only an arbitrary optical carrier( e.g., arbitrary single optical carrier) of the plural optical carriers. For example, such optical modulation is performed in the case of transmitting information such as gain and S/N ratio obtained as the result of processing in an optical repeater including an optical amplifier to a rear-stage optical repeater by means of an arbitrary optical carrier as a supervisory signal. In such a case, a conventional method includes the steps of spatially separating a desired optical carrier from wavelength division multiplexed light by using an optical demultiplexer, next modulating this optical carrier, and finally combining this optical carrier and the other optical carriers to obtain wavelength division multiplexed light again. In this manner, the conventional method has a problem that optical modulation of an arbitrary optical carrier in wavelength division multiplexed light cannot be easily performed. Furthermore, there arises another problem that many optical elements for optical demultiplexing and optical multiplexing are required to meet the a requirement of devices having complex configurations.